Liquid crystal displays (LCDs) have recently enjoyed wide use because of small thickness, light weight, and low power consumption. An LCD includes a liquid crystal cell and a polarizing plate. A polarizing plate is usually composed of a pair of protective films and a polarizing film and obtained by stretching a polyvinyl alcohol film dyed with iodine to make a polarizing film and laminating a protective film to both sides of the polarizing film. A transmissive LCD has a polarizing plate on both sides of a liquid crystal cell and may further have one or more optical compensation films.
As LCDs do not produce light themselves, they need a surface light source. Backlight type LCDs are widely employed, in which a light diffusing member and a light collecting member, such as a diffuser sheet and a prism sheet, are disposed between a liquid crystal cell and a light source to establish a uniform surface light source system. A cold cathode fluorescent lamp or a light emitting diode is used as a light source. Some LCDs employ an edge light system in which a light source is placed along the edge of a light guide film and combined with a diffuser sheet, a prism sheet, and the like to make a surface light source system. As described, a backlight system is generally composed of a linear or point light source the light from which is converted to uniform surface light by use of a diffuser sheet.
A schematic view of a conventional LCD equipped with a backlight system for obtaining a surface light source is illustrated in FIG. 7. The conventional LCD includes, in the order described, a light source 52 having linear light sources 56, a first light diffuser sheet 58, a light collecting sheet 54, a second light diffuser sheet 60, and a liquid crystal panel 72. The liquid crystal panel 72 includes a liquid crystal cell 70 and a pair of polarizing plates 66. Each polarizing plate 66 is composed of a polarizing film 62 and a pair of protective films 64.
Light emitted from the light source 52 is diffused through the first diffuser sheet 58 and enters the light collecting sheet 54, where it is collected toward the front side to provide increased front brightness. The second diffuser sheet 60 in front of the light collecting sheet 54 widens within a predetermined range the viewing angle once narrowed by the light collecting sheet 54.
By disposing the diffuser sheet on both sides of the light collecting sheet 54, unevenness of brightness is reduced, and in-plane uniformity of display qualities is achieved. This configuration is also effective in preventing incident light from interfering with pixels of the liquid crystal cell and in scattering side lobe generated in prisms, thereby to prevent occurrence of an interference fringe such as Moire.
In recent years, reduction in the number of linear light sources 56 used in the light source 52 has been attempted to reduce the number of members making up an LCD as well as power consumption. With a decrease in thickness of LCDs, the distance between the light source 52 and each diffuser sheet 58 or 60 has been reduced. Under these circumstances, it has now become difficult to achieve uniform light diffusion with conventional diffuser sheets. In order to shorten the distance as much as possible, it has been proposed to substitute the diffuser sheet by a light-diffusing protective film on the polarizing film of the backlight side polarizing plate making up the liquid crystal panel.
For example, JP 2000-75134A proposes a diffusing polarizing plate having a diffusing layer containing porous amorphous particles and spherical particles and having prescribed characteristics, which allows for omission of a diffuser sheet.
JP 2007-304553A discloses a light collecting sheet (prism sheet) having a transparent resin layer containing light diffusing particles dispersed therein, which prevents occurrence of moire while retaining high brightness.